In an optical communication network with optical fibers as a transmission medium, various devices (hereafter, optical communication devices) such as a transceiver to convert electric signals into optical signals and to send the signals to the optical communication network, and a receiver that is for converting the received optical signal into an electric signal, are installed.
The exit and entrance of the optical signals of these optical communication devices are connected with, for example, optical devices such as an optical attenuator that modulates intensity of optical signals to a predetermined intensity and optical isolators to prevent occurrence of a “return light” which are optical signals that has been received that flow back to a light source side. Many of these optical devices are configured with the known Faraday rotator as a main body.
The Faraday rotator is configured including a Faraday element made of magnetooptical material such as magnetic garnet single crystal and a magnetic applying measure to apply a magnetic field to this Faraday element. The magnetic applying measure can variably control the direction and the size of the magnetic field, and normally this measure is configured from permanent magnets that apply a permanent magnetic field in a vertical direction with respect to the light incident/emission surfaces of the Faraday element to make the Faraday element be magnetically saturated, and an electromagnet to apply to the Faraday element a variable magnetic field that is orthogonal to a magnetic field direction of the permanent magnets. Configurations and operations of a conventional Faraday rotator are disclosed in Patent Literatures 1 and 2 indicated below. Further, Patent Literature 3 discloses such as a conventional optical attenuator that uses a Faraday rotator and a technique to improve the characteristics of the optical attenuator.